Several controlled pressure drilling techniques are used to drill wellbores. In general, controlled pressure drilling includes managed pressure drilling (MPD), underbalanced drilling (UBD), and air drilling (AD) operations. In the Underbalanced Drilling (UBD) technique, a UBD system allows the well to flow during the drilling operation. To do this, the UBD system maintains a lighter mud-weight of drilling mud so that fluids from the formation being drilled are allowed to enter the well during the operation. To lighten the mud, the UBD system can use a lower density mud in formations having high pressures. Alternatively, the UBD system can inject an inert gas such as nitrogen into the drilling mud. During the UBD operation, a rotating control device (RCD) at the surface allows the drill string to continue rotating and acts as a seal so produced fluids can be diverted to a mud gas separator. Over all, the UBD system allows operators to drill faster while reducing the chances of damaging the formation.
In the Managed Pressure Drilling (MPD) technique, a MPD system uses a closed and pressurizable mud-return system, a rotating control device (RCD), and a choke manifold to control the wellbore pressure during drilling. The various MPD techniques used in the industry allow operators to drill successfully in conditions where conventional technology simply will not work by allowing operators to manage the pressure in a controlled fashion during drilling.
During drilling, the bit drills through a formation, and pores become exposed and opened. As a result, formation fluids (i.e., gas) can mix with the drilling mud. The drilling system then pumps this gas, drilling mud, and the formation cuttings back to the surface. As the gas rises up the borehole, the pressure drops, meaning more gas from the formation may be able to enter the wellbore. If the hydrostatic pressure is less than the formation pressure, then even more gas can enter the wellbore.
Gas traps, such as an agitation gas trap, are devices used for monitoring hydrocarbons in drilling mud at the surface so operators can evaluate hydrocarbon zones downhole. To determine the gas content of drilling mud, for example, a typical gas trap mechanically agitates mud flowing in a tank. The agitation releases entrained gases from the mud, and the released gases are drawn-off for analysis. The spent mud is simply returned to the tank to be reused in the drilling system. Unfortunately, the way that the agitator gas trap extracts gas from the drilling mud limits the reliability of its results. In addition, the total level of hydrocarbons in the mud (especially methane C1) heavily influences readings by the gas trap.
In MPD or UBD systems, the surface circulating system circulates drilling mud from the wellhead to pits. This circulating system is principally enclosed and uses a mud gas separator to remove gas from the drilling mud. The MPD or UBD systems present a number of problems for traditional surface gas detection. Unfortunately, traditional gas traps are not designed to work in enclosed pipe and do not operate under greater than ambient pressures. Therefore, any gas detection using the typical gas trap in the MPD and UBD systems must take place in the trough or at the end of the mud gas separator. In both cases, however, the gas trap produces erroneous gas signatures.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.